1. Field of the Invention
This invention relates to the art of making rotary drilling tools and more particularly to the art of drilling nonferrous metals at high feed rates, high speeds, and with deep penetration.
2. Discussion of the Prior Art
A significant problem inhibiting deep hole drilling of metals is the inability to remove chips and cutting debris fast enough so that the drill cutting edges are not clogged or jammed.
One of the earliest drill constructions used to achieve close tolerance holes in metals has been gun drills. Such drills essentially comprise a cylinder of metal having a diameter essentially the same size as the hole to be achieved, with a wide but axially extending and axially straight groove or flute along one side of the cylinder to define the cutting edges. The groove is intended to allow for metal chips to force their way out of the drilled opening therethrough by mechanical action of the generation of the chips themselves. Unfortunately, gun drills do not have good centering capability when high torque is applied; such unbalanced forces cause off-centering and cause bending which may result in scoring. Such gun drills cannot be used at high rotary speeds and are ineffective for accurate deep hole penetration.
Spiral fluted drills with multiple lands have been used to ensure greater hole centering accuracy (see U.S. Pat. No. 3,359,837). Unfortunately, spiral fluted drills rely on the mechanical nature of the chips to force their way out of the drill, up through the spiral flutes, which mechanical action cannot be carried out at high speed and high feed rates.
U.S. Pat. No. 2,912,887 discloses an improvement to gun drills and to the spiral fluted drills, which is particularly adapted for deep hole drilling. It uses double, diametrically opposed, spiral flutes for accurate deep hole drilling and uses a single spiral flute higher up the shank of the bit to facilitate the deployment of cutting fluid to assist in moving chips or cutting debris away. This disclosure is effective with ferrous workpieces which produce short brittle chips, but is ineffective when machining nonferrous workpieces, such as aluminum, that produce chips that are stringers, considerably longer, and are not easily mechanically moved through the double fluted, nonfluidized portion of the drill. If the drill of U.S. Pat. No. 2,912,887 were to be used at high rotary speeds, such as in excess of 6000 rpm and high feed rates in excess of 60 inches per minute, the drill would not be able to evacuate the chips rapidly enough and the drilling operation would jam.